1. Field of the Invention
The present invention relates to a receiving device for a spread spectrum communication.
2. Description of the Related Art
In a spread spectrum communication, a transmitting device spreads and modulates an original signal with a spreading code and transmits the modulated signal. A receiving device despreads and demodulates a received signal with the spreading code. When the receiving device despreads the received signal, the receiving device acquires synchronization of the received signal and the spreading code.
For acquiring synchronization of the spreading code, a sliding correlator can be used, for example. When the sliding correlator is used, a process for determining a synchronizing point of the received signal and the spreading code is repeated at each time when the receiving device receives a signal of one phase. Thus, it may take a long time to acquire synchronization and a response of a communication system may be reduced.
Alternatively, a matched filter can be used for acquiring synchronization. In such a case, a synchronizing point of the received signal and the spreading code is concurrently determined for the received signals of a plurality of phases. Thus, the time required for acquiring synchronization can be reduced compared with a case where the sliding correlator is used. A configuration of the matched filter corresponds to a configuration of a plurality of sliding correlators arranged in parallel. Thus, a circuit size increases in a case where the received signal is despread in an analog process, and the number of gates increases in a case where the received signal is despread in a digital process. Thus, a cost, the circuit size, and a consumption current increase. The time required for acquiring synchronization can be reduced by concurrently executing a process for auto correlating the received signal with the spreading code. However, the number of the process for the autocorrelation is not much different from a case where the sliding correlator is used.
A sliding correlator disclosed in JP-A-5-219011 counts the number of matching and mismatching of the received signal and the spreading code. The sliding correlator counts only until the receiving device acquires synchronization at a threshold level. Thereby, the number of processes can be reduced and the time for acquiring synchronization can be reduced. In the method described in JP-A-5-219011, a transmitting device transmits only a fixed-value data until the receiving device completes the acquisition. The transmitting device needs to be informed that the synchronization acquisition is completed. Thus, it takes time from when the synchronization acquisition is completed till when the receiving device receives an actual data.
In a communication system disclosed in JP-A-8-23710, a transmitting device inserts a non-spread preamble between bits of a spread transmitting signal. A receiving device detects a synchronizing point based on the non-spread preamble. In such a case, a communication time increases due to the non-spread preamble. Thus, a transmitting rate is reduced for the preamble when ordinary data is transmitted and received after the acquiring synchronization.